Apparatus for detecting the condition of switches in one transmission line

ABSTRACT

The switch condition of two switches (S1, S2) connected in parallel to a single transmission line (11) is detected by the signals output from two Schmitt triggers (10a, 10b) which are arranged to operate at different switching threshold levels by virtue of the output (WA) of one of the triggers (10a) being connected to the threshold level control input (F) of the other trigger (10b). Closure of one of the switches (S1 or S2) causes a reference voltage (UR) to the applied to the inputs of the triggers (10a, 10b) which is turn causes the upper threshold trigger (10a) to operate. Closure of both switches (S1 and S2) reduces the level of the voltage applied to the inputs of the triggers which is turn causes the lower threshold trigger to operate. Preferably, the signal input to the lower threshold trigger is converted into a current prior to application to the trigger due to the relatively small difference in threshold levels.

BACKGROUND

The present invention relates to apparatus for detecting the conditionof two switches connected to a single transmission line.

Many systems are electronically controlled using microprocessors whichare supplied with signals from external sensors. In many systems, thesensors comprise switch devices of one sort or another and it isimportant to be able to recognise the condition of each switch device.Ideally, each switch device would be connected to the microprocessor bya respective transmission line or else a bus would interconnect allswitch devices and each switch device would have a unique address. Thefirst of the ideal solutions is not possible in some circumstances dueto lack of space and the second is expensive and requires a specialconstruction of control system. There thus occurs in practice asituation where more than one switch device is connected to a processorby a single transmission line.

It is possible to evaluate the conditions of two switch devices on asingle transmission using digital evaluation techniques in a digitalchannel, but often all digital channels in a microprocessor are fullyutilised.

The present invention provides an arrangement whereby an analog channelcan be used to evaluate the condition of each of two switches on asingle analog transmission line.

The advantages of this arrangement are that no analog channel into theprocessor is required and hence no A/D conversion. Processor inputsdesigned for other tasks can be utilised with a resulting in saving ofdevelopment time and cost.

DRAWINGS

In order that the present invention be more readily understood, anembodiment thereof will now be described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 is a diagram of circuitry according to the present invention; and

FIGS. 21-d show wave form s with a common horizontal time axis, whichare helpful in explaining the operation of the circuitry shown in FIG.1.

DETAILED DESCRIPTION

The embodiment shown in FIG. 1 of the drawings relates to a controlcircuit based on a hybrid circuit device 10 including two Schmitttrigger devices 10a, 10b, the first of which has an input WE and anoutput WA, and the second of which has an input GE and an output GA. Thesecond Schmitt trigger 10b has a control input F. If the input of F islow then the input impedance of the input GE is high; otherwise a highvoltage on F forces GE to a low input impedance. The output from WA isfed back to the input F of the other Schmitt trigger to ensure that thesecond Schmitt trigger only switches when the output of the firstSchmitt trigger 10a is low.

An analog transmission line 11 is connected to the input WE of the firstSchmitt trigger via a resistor combination R4, R5 and to the input GE ofthe second Schmitt trigger via a differentiating capacitor C2 and aresistor divider network R6, R7 whose values set the operating point ofthe second Schmitt trigger. Two switches S1 and S2 are connected inparallel via respective series resistors R1 and R2 to the transmissionline 11. A series connected diode D1 and resistor R0 connect thetransmission line to a source of reference potential UR e.g. at 5 volts,in order to establish a reference voltage level on the transmission linewhich is independent of battery voltage. However, the transmission line11 is also connected to battery voltage UB via resistor R3 in order toachieve a large voltage excursion beyond the reference voltage levelwhen both switches S1 and S2 are open. It has been found that optimumvoltage excursions of the voltage on the transmission line occurs whenR0, R1 and R2 are equal in value.

Referring now to FIG. 2, this shows four waveform diagrams 2a-2d, theuppermost of which 2a represents the voltage excursions UIN at the inputWE of the first Schmitt trigger; the next waveform 2b represents theoutput voltage U from the output WA of the first Schmitt trigger; thenext waveform 2c shows the waveform of the time differential of thewaveform U_(IN) i.e. the input to GE as the switches S1 and S2 areclosed and opened sequentially; and the final waveform 2d shows theoutput U from the output GA of the second Schmitt trigger.

In the initial condition of the circuitry shown in FIG. 1, the twoswitches S1 and S2 are assumed to be both open in which case and, asshown in 2a, the voltage on the transmission line 11 approximates tobattery voltage UB. As soon as one of the switches is closed, thevoltage at the input WE of the first Schmitt trigger drops to areference level determined by the value of the reference voltage and thevalues of the resistors R1 or R2 and R0. As indicated in 2b, the outputvoltage from the output WA of the first Schmitt trigger drops but theoutput voltage from the output GA of the second Schmitt trigger isunaffected as shown in 2d.

When both switches S1 and S2 are closed, the voltage waveform 2a dropsstill further by an amount delta U due to the parallel combination ofresistors R1 and R2. This has no effect on the output voltage at theoutput WA of the first Schmitt trigger but now causes the voltage at theoutput GA of the second Schmitt trigger to drop as shown in 2d.

The voltage excursion delta U when both switches are closed is onlyrelatively small, delta U approximately equals 0.8 volts. The capacitorC2 converts the small voltage excursion into a switching current asindicated in diagram 2c in order to avoid problems due to narrowing ofthe useful switching excursion caused by earth potential offset andswitching tolerances had a direct voltage coupling been used as in theinput to the first Schmitt trigger.

As soon as one of the switches S1 or S2 is opened, the output from thesecond Schmitt trigger GA goes high as shown on the right-hand side of2d due to the removal of the offset voltage delta U from thetransmission line 11. When the second switch is opened, the circuitreturns to its initial conditions as indicated on the right hand side ofall the diagrams.

As will be seen from FIG. 1, the output WA from the first Schmitttrigger indicates when either S1 or S2 is closed and the output GA fromthe second Schmitt trigger indicates when both the switches S1 and S2are closed. Thus, a binary evaluation of the two switch inputs isobtained and can be evaluated by a microprocessor without the need of aA/D converter. There are many instances where there is an unambiguousswitching sequence which can be interpreted by the microprocessor i.e.first S1 is closed, then S2 is closed then S2 is opened, and finally S1is opened.

I claim:
 1. Apparatus having two external sensors in the form of a firstswitch device (S1) and a second switch device (S2), each responding toexternal influences by shifting between a closed switch condition and anpen switch condition,including means for obtaining two binary outputsignals which together indicate the switch conditions of both of saidswitch devices using only a single analog transmission line (11) towhich said first and second switch devices (S1, S2) are connected, inparallel, via respective resistances (R1, R2), said means includingfirst and second trigger switching devices (10a, 10b) having respectiveinputs (WE, GE) which are connected to the transmission line, andrespective outputs (WA, GA) which provide said binary output signals,means for applying a reference voltage (UR) to the inputs of saidtrigger switching devices (10a, 10b), means for setting the threshold ofthe two trigger switching devices (10a, 10b), and a differentiatingnetwork (C2, R6, R7), wherein the input (WE) of said first riggerswitching device is connected directly to said single analogtransmission line (11); the input (GE) of said second trigger switchingdevice is connected via said differentiating network to said singleanalog transmission line (11); said second trigger switching device alsohas a control or feedback input (F) which is connected to the output(WA) of the first trigger switching device (10a) to ensure that thesecond trigger switching device (10b) only switches when the binaryoutput signal of the first rigger switching device is low; whereby thebinary output signal of the first triger switching device indicates wheneither of said sensors (S1, S2) is in a closed switch condition, and thebinary output signal of said second trigger switching device indicateswhen both of said sensors (S1, S2) are in a closed switch condition,said two binary output signals thereby together defining the respectiveswitch conditions of both sensors without any additionalanalog-to-digital conversion circuitry.
 2. Apparatus according to claim1,further comprising an additional power supply (U_(B)) having an outputwhich is applied to said transmission line (11) to pull up a voltage onthe transmission line (11) to a higher level when said switch devices(S1, S2) are in said open switch condition.
 3. Apparatus according toclaim 1,wherein the trigger switching devices (10a, 10b) are Schmitttriggers.
 4. Apparatus according to claim 2,wherein the triggerswitching devices (10a, 10b) are Schmitt triggers.
 5. Apparatusaccording to claim 1,wherein said differentiating network includes adifferentiating capacitor which couples a voltage on the transmissionline (11) to the input (GE) of the second trigger switching device(10b).
 6. Apparatus according to claim 2,wherein said differentiatingnetwork includes a differentiating capacitor which couples a voltage onthe transmission line (11) to the input (GE) of the second triggerswitching device (10b).
 7. Apparatus according to claim 3,wherein saiddifferentiating network includes a differentiating capacitor whichcouples a voltage on the transmission line (11) to the input (GE) of thesecond trigger switching device (10b).
 8. Apparatus according to claim4,wherein said differentiating network includes a differentiatingcapacitor which couples a voltage on the transmission line (11) to theinput (GE) of the second trigger switching device (10b).
 9. Apparatusaccording to claim 1,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 10. Apparatusaccording to claim 2,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 11. Apparatusaccording to claim 3,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 12. Apparatusaccording to claim 4,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 13. Apparatusaccording to claim 5,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 14. Apparatusaccording to claim 6,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 15. Apparatusaccording to claim 7,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 16. Apparatusaccording to claim 8,characterized in that the means for applying thereference voltage (UR) to the transmission line (11) comprises a voltagesource (UR) and a series-connected resistance (R0) and diode (D1)connected between said source and said transmission line.
 17. Apparatusaccording to claim 9,wherein the resistance values of theseries-connected resistance and the respective resistances (R1, R2)which connect the two switch devices (S1, S2) to the transmission lineare all the same.